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A termite "cathedral" mound produced by a termite colony: a classic example of emergence.

Emergence is the process of complex pattern formation from simpler rules.

This can be a dynamic process (occurring over time), such as the evolution of the human body over thousands of successive generations; or emergence can happen over disparate size scales, such as the interactions between a great number of neurons producing a human brain capable of thought (even though the constituent neurons are not individually capable of thought). The original term was "categorial novum" coined by Nicolai Hartmann.

For a phenomenon to be termed emergent it should generally be unpredictable from a lower level description. At the very lowest level, the phenomenon usually does not exist at all or exists only in trace amounts: it is irreducible. Thus, a straightforward phenomenon such as the probability of finding a raisin in a slice of cake growing with the portion-size does not generally require a theory of emergence to explain. It may, however, be profitable to consider the "emergence" of the texture of the cake as a relatively complex result of the baking process and the mixture of ingredients.

Like intelligence in AI,it is a central concept in
complex systems yet is hard to define and very controversial. There is no scientific consensus about what weak and strong forms of emergence are, or about how much emergence should be relied upon as an explanation in general. It seems impossible to unambiguously decide whether a phenomenon should be considered emergent.

Further, "emergent" is not always a deeply explanatory label even when it is agreed on: the more complex the phenomenon is, the more intricate are the underlying processes, and the less effective the word emergence is alone. In fact, calling a phenomenon emergent is sometimes used in lieu of a more meaningful explanation. See also: self-organization.

Sometimes the term is used in the colloquial meaning of emersion or appearance.

Contents

One possible graphical view of the principle of emergence as described in Roger Lewin's book about "Complexity - Life at the Edge of Chaos", Figure 10, page 189.

An emergent behaviour or emergent property can appear when a number of simple entities (agents) operate in an environment, forming more complex behaviours as a collective. If emergence happens over disparate size scales, then the reason is usually a causal relation across different scales. In other words there is often a form of top-down feedback in systems with emergent properties. These are two of the major reasons why emergent behaviour occurs: intricated causal relations across different scales and feedback. The property itself is often unpredictable and unprecedented, and may represent a new level of the system's evolution. The complex behaviour or properties are not a property of any single such entity, nor can they easily be predicted or deduced from behaviour in the lower-level entities: they are irreducible. No physical property of an individual molecule of air would lead one to think that a large collection of them will transmit sound. The shape and behaviour of a flock of birds or school of fish are also good examples.

One reason why emergent behaviour is hard to predict is that the number of interactions between components of a system increases combinatorially with the number of components, thus potentially allowing for many new and subtle types of behaviour to emerge. For example, the possible interactions between groups of molecules grows enormously with the number of molecules such that it is impossible for a computer to even count the number of arrangements for a system as small as 20 molecules.

On the other hand, merely having a large number of interactions is not enough by itself to guarantee emergent behaviour; many of the interactions may be negligible or irrelevant, or may cancel each other out. In some cases, a large number of interactions can in fact work against the emergence of interesting behaviour, by creating a lot of "noise" to drown out any emerging "signal"; the emergent behaviour may need to be temporarily isolated from other interactions before it reaches enough critical mass to be self-supporting. Thus it is not just the sheer number of connections between components which encourages emergence; it is also how these connections are organised. A hierarchical organisation is one example that can generate emergent behaviour (a bureaucracy may behave in a way quite different to that of the individual humans in that bureaucracy); but perhaps more interestingly, emergent behaviour can also arise from more decentralized organisational structures, such as a marketplace. In some cases, the system has to reach a combined threshold of diversity, organisation, and connectivity before emergent behaviour appears.

Unintended consequences and side effects are closely related to emergent properties.
Luc Steels writes about a system with "emergent functionality" in his paper
Towards a Theory of Emergent Functionality:
"A component has a particular functionality but this is not recognizable as a subfunction of the global functionality. Instead a component implements a behavior whose side effect contributes to the global functionality [...] Each behavior has a side effect and the sum of the side effects gives the desired functionality". In other words, the global or macroscopic functionality of a system with "emergent functionality" is the sum of all "side effects", of all emergent properties and functionalities.

Systems with emergent properties or emergent structures may appear to defy entropic
principles and the second law of thermodynamics, because they form and increase order despite
the lack of command and central control. This is possible because open systems can
extract information and order out of the environment.

Emergence helps to explain why the fallacy of division is a fallacy. According to an emergent perspective, intelligence emerges from the connections between neurons, and from this perspective it is not necessary to propose a "soul" to account for the fact that brains can be intelligent, even though the individual neurons of which they are made are not.

Emergent behavior is also important in games and game design. For example, the game of poker, especially in no limit forms without a rigid betting structure, is largely driven by emergent behavior. For example, no rule requires that any player should fold, but usually many players do. Because the game is driven by emergent behavior, play at one poker table might be radically different from that at another, while the rules of the game are exactly the same. Variations of games that develop are examples of emergent metaplay, the predominant catalyst of the evolution of new games.

In the field of videogames, examples of emergence can be found in games such as Sim City, The Legend of Zelda and the new "open world" game concept such as GTA and The Godfather. Also in non-commercial products such as Conway's Game of Life and others attempts at making software through evolutionary means.

Emergent structures are patterns not created by a single event or rule. There is nothing that commands the system to form a pattern, but instead the interactions of each part to its immediate surroundings causes a complex process which leads to order. One might conclude that emergent structures are more than the sum of their parts because the emergent order will not arise if the various parts are simply coexisting; the interaction of these parts is central.

A biological example is an ant colony. The queen does not give direct orders and does not tell the ants what to do. Instead, each ant reacts to stimuli in the form of chemical scent from larvae, other ants, intruders, food and build up of waste, and leaves behind a chemical trail, which, in turn, provides a stimulus to other ants. Here each ant is an autonomous unit that reacts depending only on its local environment and the genetically encoded rules for its variety of ant. Despite the lack of centralized decision making, ant colonies exhibit complex behavior and have even been able to demonstrate the ability to solve geometric problems. For example, the ant colonies routinely find the maximum distance from all colony entrances to dispose of dead bodies.

Besides emergence in ant colonies, which is like other emergent structures
in social insects mainly based on pheromones and chemical scents, emergence
can be observed in swarms and flocks. Flocking is
a well-known behavior in many animal species from swarming locusts to
fish and birds. Emergent structures are a favorite strategy
found in many animal groups: colonies of ants,
piles of termites, swarms of bees, flocks of birds,
herds of mammals, shoals/schools of fish, and packs of wolves.

Emergent structures can be found in many natural phenomena,
from the physical to the biological domain.
The spatial structure and shape of galaxies is an emergent property, which characterizes the large-scale distribution of energy and matter in the universe. Weather phenomena with
a similar form such as hurricanes are emergent properties, too. Many speculate
that consciousness and life itself are emergent properties of a
network of many interacting neurons and complex molecules, respectively.

Life is a major source of complexity, and evolution is the major principle or driving force behind life. In this view, evolution is the main reason for the growth of complexity in the natural world. If we speak of the emergence of complex living beings and life-forms, we refer therefore to processes of sudden changes in evolution.

There is also a view that the beginning and development of evolution itself can be regarded as an emergent property of the laws of physics in our universe, or contrary to this the opposite view that the laws of physics have like their constituents emerged during the course of time (in which case "evolution" and "emergence" would be the most fundamental principles in the universe).

Emergent processes or behaviours can be seen in many places, from any multicellular biological organism to traffic patterns, cities or organizational phenomena in computer simulations and cellular automata. The stock market is an example of emergence on a grand scale. As a whole it precisely regulates the relative prices of companies across the world, yet it has no leader; there is no one entity which controls the workings of the entire market. Agents, or investors, have knowledge of only a limited number of companies within their portfolio, and must follow the regulatory rules of the market. Through the interactions of individual investors the complexity of the stock market as a whole emerges.

The World Wide Web (WWW) is a popular example of a decentralized system exhibiting emergent properties. There is no central organization rationing the number of links, yet the number of links pointing to each page follows a power law in which a few pages are linked to many times and most pages are seldom linked to. A related property of the network of links in the world wide web is that almost any pair of pages can be connected to each other through a relatively short chain of links. Although relatively well known now, this property was initially unexpected in an unregulated network. It is shared with many other types of networks called small-world networks.

Emergent structures appear at many different levels of organization or as spontaneous order. Emergent self-organization appears frequently in cities where no planning or zoning entity predetermined the layout of the city. The interdisciplinary study of emergent behaviours is not generally considered a homogeneous field, but divided across its application or problem domains.

In physics, emergence is used to describe a property, law, or phenomenon which occurs at macroscopic scales (in space or time) but not at microscopic scales, despite the fact that a macroscopic system can be viewed as a very large ensemble of microscopic systems. Some examples include:

Color. Elementary particles such as protons or electrons have no color; it is only when they are arranged in atoms that they absorb or emit specific wavelengths of light and thus be said to have a color. (Note that while quarks have a characteristic which has been labeled color charge by physicists, this terminology is merely figurative and has no actual relation with the everyday concept of color.)

In distinction from the behavioural sciences, an emergent property need not be more complicated than the underlying non-emergent
properties which generate it. For instance, the laws of thermodynamics are remarkably simple, even if the laws which govern the interactions between component particles are complex. The term emergence in physics is thus used not to signify complexity, but rather to distinguish which laws and concepts apply to macroscopic scales, and which ones apply to
microscopic scales.

Note that the term "emergence" is generally not well received in academia, especially among those involved in this area. The term is considered to be meaningless and ambiguous, much like many terms in new (one might say "emerging") fields in web technology.